BACTERIA IN WATER 



45 



of the depression. Upon the under side of a clean cover- 

 glass is placed a drop of pure water, and this is inoculated 

 with the smallest possible particle taken from one of the 

 colonies of the gelatine plate on the end of a sterilised 

 platinum wire. The cover-glass is then placed upon the 

 ring of vaseline, and the drop hangs into the space of the 

 depression. Thus is obtained a view of the organisms in a 

 freely moving condition, if they happen to be motile bac- 

 teria. As a matter of practice the hollow slide may be 

 dispensed with, and an ordinary slide used. 



With regard to staining, it will be undesirable here to 

 dwell at length upon the large number of methods which 



DRYING STAGE FOR FIXING FILMS 



have been adopted. The "single stain " may be shortly men- 

 tioned. It is as follows : A clean cover-glass is taken (cleaned 

 with nitric acid and alcohol, or bichromate of potash and 

 alcohol), and a drop of pure sterilised water placed upon it. 

 This is inoculated with the particle of a colony on the end 

 of a platinum needle, and a scum is produced. The film is 

 now " fixed " by slowly drying it over a flame. When the 

 scum is thus dried, a drop of the selected stain (say gentian, 

 violet) is placed over the scum and allowed to remain for 

 varying periods : sarcince about thirty seconds ; for many of 

 the bacilli three or four minutes. It is then washed off with 

 clean water, dried, and mounted in Canada balsam. The 

 organisms will now appear under the microscope as violet 

 in colour, and will thus be clearly seen. 



The " double staining" is adopted when we desire to 



